Examining Expert Evidence

This article considers how to deal with expert evidence when the other side discloses it during the course of litigation. The issue is that many advocates, particularly those who are junior (or those like myself are commencing their second sixth), may not challenge the expert sufficiently in order to test his evidence. This might be for a variety of reasons but may include: a lack of knowledge about the subject area, a feeling of acquiescence in the sense that the expert must be right or a feeling of inferiority compared to the expert who may have many years of experience. These feelings may all lead to an expert going unchallenged or not adequately challenged during cross-examination. The advocate must remember one of the key purposes of cross-examination is to put your client’s case. In cases where there are experts this duty to put your case requires the expert and his underlying evidence to be robustly challenged. The way in which this is done depends on the expert and their evidence but this article aims to provide some general practical tips for those who cross-examine experts.

An expert is defined in CPR 35.2 as a person “who has been instructed to give or prepare evidence for the purpose of court proceedings.” Herein lies the first potential problem with expert reports. He has been ‘instructed’. Unless the expert is jointly instructed, each party may have an agenda of proving their respective cases (to the extent that is ethically permissible). Instructions to experts may be written with this agenda in mind and of course the expert is only obliged to answer the questions that are put to them. If not provided within the report itself, the advocate should request to see the instructions that the expert was given as this may provide a ground for challenge if the underlying instructions are written in a way as to procure a certain answer or if there are certain obvious questions that have not been asked.


The advocate needs to be aware of the potential for bias, both conscious and unconscious. The expert may have a private agenda i.e. he may seek support for a particular cause; or if acting regularly for a particular client he may be eager to please so as not to lose the business of that client. There are many recent cases where the lack of independence by the expert has resulted in deficiencies in evidence. In Liverpool Victoria Insurance Company v Zafar [2019] EWCA Civ 392 the expert was sentenced for contempt of court as the court found that “… the fact that he or she is acting corruptly and makes the relevant false statement for reward, will make the case even more serious; but it will be a serious contempt of court even if the expert witness acts from an indirect financial motive (such as a desire to obtain more work from a particular solicitor or claims manager), or without any financial motivation at all, and even if the expert witness stands to gain little financial reward by it.” This demonstrates the seriousness with which the court takes experts who are not complying with their duty to the court. The advocate must be alive to the need to consider any potential bias that may taint the evidence and be prepared to challenge this in oral evidence.

There are a number of routes by which the evidence of the expert can be robustly tested. I have selected some of the more general examples but there are other potential routes of challenge that may be applicable on the facts.

Qualifications and Experience

The advocate should review the expert’s qualifications and experience. The advocate must consider whether his expertise is pertinent to address the issues in the case. For example, a cardiologist would not be best placed to give evidence on the mental health of an individual, but a psychologist may be. Also the advocate should consider what the expert’s experience actually is – how long did he spend in practice? Is he still practising in the field of his expertise or is he now in academia or a different field? This is important as this could be used to determine whether his knowledge is actually up-to-date with the current practice in the relevant field. This may or may not be evident on the face of the report. If it is not, the advocate in preparation of the case should get a copy of the expert’s CV and assess whether any of these challenges can be legitimately made against the expert.

Underlying Facts

An expert’s report is only reliable if it is based on the correct facts and/or data. It is therefore important for the advocate to be clear on the facts and to assess whether it matches those that the expert recites in his report as being the underlying facts. If there are errors then the expert may have been careless when preparing his report or may simply have not known all the material facts. If the advocate is able to identify errors this could result in the expert changing his evidence in light of the correct facts or, at the very least, it may undermine his credibility. From the expert reports that I have encountered thus far it appears increasingly common in the medical field for the expert not to have actually examined the individual who they are diagnosing. There may be grounds for challenging the validity of, or at least the reliability of, their diagnosis in light of this. Also, the advocate needs to guard against the risk of expert’s giving opinions about the dispute in the case (e.g. giving opinions on liability) rather than giving evidence within the remit of their professional expertise.


The decision maker may not be able to assess the reliability of the technical evidence so may naturally defer to the expert. In order to assist the court, the advocate needs to thoroughly understand the report and any associated jargon contained within it. Research can assist with this but the advocate should also use their own expert to explain the terminology in layman’s terms. Only when the advocate fully understands the report can he begin to consider whether the conclusions are logical, assumptions are justified, data is reliable, and if the data been tested effectively.

Careful analysis of the words used by the expert may sometimes be required. Experts may use words to make general statements rather than answering the direct question. For example, phrases such as “no material difference” or “consistent with” are words that can be problematic as it may not be clear what the expert’s conclusion actually is. These words may have been used carelessly; or (worse) may have been chosen in order to decrease the impact of his conclusions. For example, what does “no material difference” actually mean? This should be thoroughly tested in cross-examination.


It is well known that figures can be manipulated to produce different results depending on the aim that you are seeking to achieve. It is therefore of critical importance that careful consideration is given to statistics and what they actually show. For example, I was involved in a case where the statistics had been produced using a small sample but then 000 had been added on to the end of the figures in order to give the appearance of a much bigger sample size. It is therefore important for the advocate to analysis the statistics and to consider if they are reliable.


In summary, when preparing a case the advocate should seek to understand the evidence that is being given, the factual matrix and the aspects upon which the expert evidence should be tested. Only in doing this can the advocate assist the court and effectively put his client’s case. Of course, this is subject to the normal commandment that thou ought only to cross-examine if it is going to advance your client’s case or undermine your opponent’s case. For those wanting to know more about expert evidence there is a wealth of information available online including the ICCA’s paper on ‘Guidance on the preparation, admission and examination of expert evidence.’

None of what has been written above is intended to be a generalisation on the excellent work and expertise that many experts provide but the advocate must be alive to the issues and be prepared to challenge them when necessary.


By Laura Stockin, pupil barrister

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